Abstract
AbstractWe perform a z-band survey for an optical counterpart of the binary neutron star coalescence GW170817 with Subaru/Hyper Suprime-Cam. Our untargeted transient search covers 23.6 deg2 corresponding to the 56.6% credible region of GW170817 and reaches the 50% completeness magnitude of 20.6 mag on average. As a result, we find 60 candidate extragalactic transients, including J-GEM17btc (also known as SSS17a/DLT17ck). While J-GEM17btc is associated with NGC 4993, which is firmly located inside the 3D skymap of GW170817, the other 59 candidates do not have distance information in the GLADE v2 catalog or NASA/IPAC Extragalactic Database. Among 59 of the candidates, 58 are located at the center of extended objects in the Pan-STARRS1 catalog, while one candidate has an offset. We present location, z-band apparent magnitude, and time variability of the candidates and evaluate the probabilities that they are located within the 3D skymap of GW170817. The probability for J-GEM17btc is 64%, which is much higher than for the other 59 candidates (9.3 × 10−3–2.1 × 10−1%). Furthermore, the possibility that at least one of the other 59 candidates is located within the 3D skymap is only 3.2%. Therefore, we conclude that J-GEM17btc is the most likely and distinguished candidate to be the optical counterpart of GW170817.
Highlights
The existence of gravitational waves (GWs) is predicted in the theory of general relativity
The large diversity in the depth of images taken on August 19 causes the shallow dependence of completeness on the point spread function (PSF) magnitude of artificial sources
28-7 some of them could be active galactic nuclei (AGN) or indistinguishable residuals resulting from different instrumental signatures between PS1 and HSC, but we conservatively consider them as candidates
Summary
The existence of gravitational waves (GWs) is predicted in the theory of general relativity. LIGO and Advanced Virgo subsequently detect three GW signals and one candidate signal, all from the coalescence of black hole binaries (Abbott et al 2016b, 2016c, 2017b, 2017c). These discoveries open the era of “gravitational wave astronomy.”. The non-detection of EM counterparts is not surprising because the four GWs originate from mergers of black holes, several theoretical studies try to explain the putative Fermi/GBM emission (e.g., Yamazaki et al 2016). GW170817 appeared at a position close to the Sun, the first significant alert of a binary NS coalescence and the narrow sky localization area initiated many EM follow-up observations (Abbott et al 2017a). Throughout the paper, we correct the Galactic reddening (Schlafly & Finkbeiner 2011), and all the magnitudes are given as AB magnitudes
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